1. Field of the Invention
This invention is in the field of chemical processes and apparatus; more specifically, the electrochemical synthesis and production of Group IV and V volatile hydrides, and a reactor for carrying out the synthesis. The synthesis and the reactor are designed to produce high purity hydrides directly in a chemical vapor deposition reactor.
2. Art Related to the Invention
High purity gases are required for semiconductor fabrication and doping. Often these gases are dangerously toxic. Commercial compressed gas cylinders store gas at several thousand pounds per square inch pressure and contain one to ten pounds of gas. Hence, centralized production, transportation and storage of these materials presents a hazard to those working with them.
The process and apparatus of this invention provides for these dangerous gases to be generated when they are needed and where they are needed. This provides a safe alternative to the use of cylinders of compressed gas in a semiconductor fabrication reactor, by greatly reducing the amount of gas which must be kept on hand.
The following references disclose processes for producing these gases by chemical methods:
Cotton and Wilkinson, "Advanced Inorganic Chemistry", Fourth Ed. Wiley Interscience, 1980 and Brauer "Preparative Inorganic Chemistry", Academic Press 1963 teach that the Group IV and V hydrides can be produced by chemical reduction of electropositive compounds of the desired product gas element with acids or by the reduction of the halides with LiAl.sub.4 or NaBH.sub.4. For example: EQU Na.sub.3 P+3 H.sub.2 O.fwdarw.PH.sub.3 +NaOH EQU Mg.sub.3 Sb.sub.2 +6 HCL.fwdarw.2 SbH.sub.3 +3 MgCl.sub.2 EQU Na.sub.3 As+3 NH.sub.4 Br.fwdarw.AsH.sub.3 +3 NaBr+3 NH.sub.3 EQU Mg.sub.2 Ge+4 NH.sub.4 Br.fwdarw.GeH.sub.4 +2 MgBr.sub.2 +4 NH.sub.3 EQU GeCl.sub.4 +LiAlH.sub.4 .fwdarw.GeH.sub.4 +LiCl+AlCl.sub.3.
These gases can also be prepared by the electrochemical reductions: EQU Sb+3 H.sub.2 O+3e.fwdarw.SbH.sub.3 +3 OH- EQU As+3 H.sub.2 O+3e.fwdarw.AsH.sub.3 +3 OH- EQU Ge+4 H.sub.2 O+4e.fwdarw.GeH.sub.4 +4 OH- EQU P+3 H.sub.2 O+3e.fwdarw.PH.sub.3 +3 OH-.
In addition, dissolved ionic precursors can be used such as: EQU H.sub.2 PO.sub.2 -+5H++4e.fwdarw.PH.sub.3 +2 H.sub.2 O.
Salzberg, J. Electrochem. Soc. 101, 528 (1964) discloses the electrochemical formation of stibine at an antimony cathode. Lloyd, Trans. Faraday Soc. 26, 15 (1930) and Salzberg J. Electrochem. Soc. 107, 348 (1960) disclose the preparation of high purity arsine at an arsenic cathode. Spasic, Glas. Hem.. Drus. Beograd. 28, 205 (1963) discloses the electrochemical production of germanium hydride.
E. W. Haycock and P. R. Rhodes, U.S. Pat. No. 3,404,076 disclose a method for the electrolytic preparation of volatile hydrides. Gordon and Miller in U.S. Pat. No. 3,109,785 and U.S. Pat. No. 3,109,795, Miller and Steingart, U.S. Pat. No. 3,262,871, and Miller U.S. Pat. No. 3,337,443 disclose electrolytic methods for the production of phosphine. Porter in U.S. Pat. No. 4,178,224 discloses an electrochemical method for the synthesis of arsine gas. His method utilizes a dissolved arsenic salt with an oxygen evolving anode. With this method, the arsine concentration is limited to less than 25%. Another limitation of Porter's method is the need to balance pressures and liquid levels in the divided anode and cathode sections of the electrochemical cell. This requires an inert gas supply to the cell.
The application of a potentiostat to an electrolytic cell is ably discussed by Bard and Faulkner in Electrochemical Methods, John Wiley and Sons, 1978.